1. Field of the Invention
This invention relates to implantable defibrillator systems, and particularly to the electrodes and pulse generators used with such systems.
2. Description of the Prior Art
The departure of the heart from normal action to uncoordinated and ineffectual contractions "fibrillation" can lead to death within minutes unless corrected. One method of treatment to restore the normal heart action involves passing electrical current through the heart muscle. The effectiveness of such treatment is dependent on a number of factors, including the location of the electrodes used to apply the electrical current, the shape of the electrodes, and the magnitude, timing, and waveform of the current. While all these factors are significant, a fundamental problem of all such electrical treatments arises from the fact that they all require large currents, several amperes to accomplish defibrillation. And, because the heart muscle typically presents an electrical impedance in the range of 40 to 100 ohms, signal amplitudes of several hundred volts are required to obtain the necessary current. The requirements for relatively high voltage and several-ampere currents combine to place great importance on efficient, low-resistance electrode arrangements for delivering the defibrillation signal to the heart. Ideally the electrode would have no resistance itself and would be placed directly against the heart muscle to avoid the voltage drop across the tissue that surrounds the heart.
Various approaches to the optimal electrode have been attempted. For example, the epicardial-patch electrodes comprise conductive and relatively large-area elements stitched directly onto the exterior of the heart itself. While this approach is satisfactory from the electrical standpoint, the attachment of the electrodes requires a major surgical procedure, such as opening the chest cavity and exposing the heart, as depicted schematically in FIG. 1. Aside from the danger that such surgery presents to all patients, many patients who require this treatment are in such poor condition that this procedure presents an unacceptable risk. In situations where such radical surgery is inappropriate, other, less effective, electrode configurations have been used. For example, the transvenous technique utilizes a conducting filament threaded through an opening in a vein, and into the heart interior. When the filament coils up in a heart chamber, ideally against the chamber wall, a relatively large-area contact to the cardiac muscle can be made. This approach requires that two such electrodes be used, one in the right-atrium (RA) position or in the nearby superior vena cava (SVC) position, and the other placed at the right-ventricular-apex (RVA) position. Despite the fact that transvenous electrodes can be inserted with a relatively simple surgical procedure, they have a serious shortcoming. Because of the design constraints that permit them to be threaded through the blood vessels, they cannot be depended upon to make adequate contact with the interior wall of the heart, and therefore they sometimes do not direct adequate current through a sufficient portion of the heart-muscle volume to achieve defibrillation.
Another option is to combine a transvenous electrode with a subcutaneous patch (SUB) in the fashion described in U.S. Pat. No. 4,817,608 to Shapland, and in U.S. Pat. No. 4,953,551 to Mehra. This approach implants a shallow, just-under-the-skin conductive element of appreciable area on the patient's left side to serve as an electrode, as illustrated in FIG. 2. Since the patch is not directly on the heart, current must pass through the intervening body tissue and fluid to reach the heart. The resistance of the intervening tissue and fluid requires the application of a higher voltage to achieve the desired current through the heart muscle, and the passage of the current through the intervening material may lead to patient discomfort. Additionally, while the surgical procedure for implanting the subcutaneous patch is relatively minor compared to that required for implantation of electrodes directly against the heart muscle, it still presents some risk to the patient. Although the subcutaneous-patch approach provides the advantage of simpler and less risky surgery, the proximity of a subcutaneous patch to the body's surface leaves the electrode relatively unprotected, and as a result, such electrodes have been subject to flexure and breakage from mishaps, and even from normal body motions.
Many patients have experienced ventricular fibrillation, or are likely to experience it. These patients are best treated by a defibrillator that is implanted in the body. Because of the relatively high voltage and substantial currents involved in treatment, the size and weight of the implanted pulse generator (PG) is an important factor. The term PG is used to identify the single package or module that contains the entire implanted defibrillator system, excluding only its electrodes and associated electrical leads. The package is usually a sealed housing made of titanium, selected for its relatively light weight and corrosion resistance. The weight of the PG is normally in excess of 200 grams, or roughly half a pound. While electrical efficiency would be better served with pectoral implantation, the size and weight of the PG usually precludes this location for cosmetic and comfort considerations, and the more spacious abdominal cavity is normally the chosen implantation site. This, of course, is in spite of the fact that PG implantation nearer the heart would result in a more compact system, with shorter leads.
Implantation of the PG nearer the heart provides the advantage of a more efficient system which in turn allows the size of the PG to be reduced. PG implantation near the heart also permits various new electrode arrangements, which are the subject of the present invention. In particular, it permits use of the metallic PG housing as an electrode (hereinafter abbreviated as "CAN". This is, in a sense, a "free" electrode since the housing is required in any case. While use of the PG enclosure as an electrode is suggested in U.S. Pat. No. 4,727,877 to Kallok, the resulting consequences were not addressed.
It is anticipated here that electrode use of the pectorally implanted PG housing will be primarily an augmentation of present systems that employ a catheter for one or more purposes. Implanting the PG involves surgery little more invasive than that required to implant a subcutaneous patch. Furthermore, it eliminates the troublesome requirement for tunneling wires under the skin that accompanies the subcutaneous patch, and the PG is also not subject to crumbling and breakage.
It is possible to use the PG enclosure as an electrode in combination with electrodes of the prior art, such as the RVA, SVC and subcutaneous-patch (SUB) electrodes. This facilitates the use of sequential defibrillation pulses having differing spatial axes, demonstrated in the prior art to reduce the amount of energy needed for defibrillation. Energy consumption is a vital concern since it is directly related to size and therefore also implantability. This is discussed in more detail by D. L. Jones, et al., "Internal Cardiac Defibrillation in Man: Pronounced Improvement with Sequential Pulse Delivery to Two Different Lead Orientations", Circulation, Volume 73, pages 484-491, March, 1986, and in their U.S. Pat. No. 4,548,203. See also, Saksena, U.S. Pat. No. 4,944,300, and Kallok, U.S. Pat. No. 4,727,877, as well as Tacker, European Patent Application 0,095,726.